(1) FIELD OF THE INVENTION
This invention relates to an aqueous epoxy resin paint composition. More particularly, the invention relates to a room temperature drying epoxy resin emulsion comprising as the main ingredient an epoxy resin emulsified in water by a non-ionic surface active agent and, incorporating therein, a curing agent and, if desired, a pigment and other additives.
(2) DESCRIPTION OF THE PRIOR ART
Recently, air pollution caused by solvents and the bad influence of solvents on the human body have become serious problems, and hence, the importance of aqueous paints is increasing. Various resin have heretofore been used for aqueous paints, and epoxy resins are often used in the field of primer paints in which such properties as rust-preventive property and chemical resistance are regarded as especially important.
In the art, the following aqueous epoxy resin compositions have heretofore been known. For example, a composition comprising (1) a polyepoxy resin and (2) a copolymer of an alkyl ester of an .alpha.,.beta.-ethylenically unsaturated monocarboxylic acid and an alkyl half ester of itaconic acid is known as an aqueous coating composition excellent in alkali resistance, adhesion and storage stability (U.S. Pat. No. 3,238,170). Further, an aqueous emulsion comprising epoxy resin and a protein acting as a protective colloid and an emulsion comprising a mixture of an amine-epoxy resin condensate and an epoxy resin are known in the art (U.S. Pat. No. 3,020,250 and U.S. Pat. No. 3,449,281).
Moreover, research work has heretofore been made on various emulsifiers for emulsifying epoxy resins in water. For example, in the art are known an aqueous emulsion of an epoxy resin in which a reaction product of a boric acid ester derived from boric acid with both an alkylene glycol and a .beta.-dialkyl-substituted amino-alkanol is used as an emulsifier (U.S. Pat. No. 3,301,804), a polyepoxide emulsion for electro-deposition in which a polyepoxide is emulsified with an emulsifying agent of the phosphate ester type (U.S. Pat. No. 3,634,348), an epoxy resin emulsion for sizing glass fibers in which a cationic emulsifying agent selected from the group consisting of imidazolines and amides and a non-ionic emulsifying agent are used in combination (U.S. Pat. No. 3,249,412), and the like.
Also various curing agents for epoxy resin emulsion compositions are known in the art. For example, as curing agents for epoxy resin compositions, there can be mentioned a polyamide reaction product derived from a polymeric fatty acid and an aliphatic polyamine containing terminal amino groups (U.S. Pat. Nos. 2,811,495 and 2,899,397), a salted amine derived from a carboxylic acid having 1 to 8 carbon atoms and a tertiary amine (U.S. Pat. No. 3,640,926), a polyamide derived from a diamine and a dicarboxylic acid (U.S. Pat. No. 3,355,409), a phenol modified polyamine (U.S. Pat. No. 3,383,347), an amino-containing polyamide prepared by reacting a polyamine with a polymerized fatty acid (U.S. Pat. No. 3,324,041), and the like.
A bisphenol type epoxy resin is ordinarily used as a vehicle of an aqueous epoxy resin paint.
The bisphenol type epoxy resin, however, has the fatal defect that the epoxy resin is crystallized when it is stored for 3 months or longer especially at low temperatures. The cause of this undesired crystallization is not yet certain, but in view of the fact that a liquid epoxy resin having a narrow molecular weight distribution range is easily crystallized at low temperatures, it is considered that the crystallization is due to arrangement of molecules having the same structure. Once crystals are formed in the epoxy resin, they grow gradually and finally, they precipitate in the form of particles to opacify the liquid resin. This phenomenon is especially likely to occur if the paint is gradually cooled to about 0.degree. C., and precipitated particles are dissolved again under heating. In short, this phenomenon is quite similar to the phenomenon in which inorganic salts are crystallized from aqueous solutions. Re-dissolving of crystals in an epoxy resin does not, in general, proceed at temperatures approximating room temperature. Accordingly, it is very difficult to restore the original state by allowing crystals in the epoxy resin to stand still as they are, and it is generally necessary to heat the crystal-containing paint to about 50.degree. C. by external heating or the like. Furthermore, in the case of an aqueous epoxy resin paint, destruction of emulsion particles is caused to occur with advance of crystallization, and when destruction of emulsion particles occurs, it is impossible to restore the origianl state even by conducting heating afterwards. Therefore, an aqueous epoxy resin paint has a fatal defect that a once crystallized paint cannot be used any more.
The epoxy resin to be used as the main component of the vehicle has the foregoing defects, and in addition, an optimum curing agent for aqueous epoxy resin compositions has not been developed.
For instance, when a polyamide resing having a high amine value is used as the curing agent, the resulting composition is defective in that gelation does not occur even after the passage of pot life, and no satisfactory results are obtained when the curing agent of this type is employed.
More specifically, even if an ordinary polyamide resin is mixed with an epoxy resin and the resulting composition is applied to a substrate, since hardly any water is evaporated from the coating film under high-humidity conditions, almost no gelatin proceeds even after the passage of pot life but partial gelation alone occurs. In an extreme case, no uniform film is obtained but a powdery film is left. This is a defect observed when a polyamide is used as the curing agent. In other words, in the case of an aqueous epoxy resin paint, it is considered that if water is present in the coating film, the activity of the curing agent is reduced, and especially under high-humidity conditions, almost no water is evaporated from the coating film and hence, gelation does not occur even after the passage of pot life.
In general, an epoxy resin paint is used as a primer paint for use in coating bridges, tanks or the like. However, when it is applied to especially close surfaces or on the inner face of a tank or the ballast tank of a ship which is in the substantially sealed state, the abovementioned defect is brought about and a sufficiently cured film cannot be obtained.
A paint excellent in rust-preventive property and chemical resistance has heretofore been prepared by using a chromate type pigment in combination with an aqueous epoxy resin composition such as mentioned above. However, in this paint including a pigment of the chromate type, chromium ions are gradually dissolved out from the coating film and there is a risk of causing environmental pollution.
As is apparent from the foregoing, the conventional aqueous epoxy resin paints have various defects and when they are used as primer paints of the natural drying type, various problems result.